1. Field of the Invention
The invention relates to a cooling passage of a component subjected to high thermal loading.
2. Discussion of Background
In fluid-flow machines, in particular gas turbines, it is quite common practice to cool components subjected to high thermal loading by integrated cooling passages, the cooling medium or cooling fluid used being air which is branched off as a partial-air flow from the working medium.
During the cooling of gas-turbine blades or combustion-chamber walls, the problem occurs that very high internal heat-transfer coefficients must be realized in order to deliver the heat flow introduced from outside to the cooling fluid and to lower the material temperature to the maximum permissible value. Special excitation mechanisms, such as, for example, ribs or pins, are therefore used for improved heat transfer.
A fundamental investigation of this problem is found in Hong, Y. J. and Hsieh, S. S. xe2x80x9cHeat transfer and friction factor measurements in ducts with staggered and in-line ribsxe2x80x9d, Journal of Heat Transfer 1993, pp 58-65. Investigations concerning the improvement of the heat transfer by a rib arrangement in a rectilinearly running cooling passage having a rectangular cross section are described therein. The test results show that a staggered rib arrangement, in which the ribs are arranged alternately on the opposite walls, compared with a non-staggered arrangement, has advantages with regard to an improved heat-transfer coefficient and a lower pressure loss of the cooling-medium flow.
In an actual application of this knowledge, a coolable blade has been designed according to EP 0 825 332 Al, on which the invention is based. The cooling passage results as a cavity between a suction-side wall and a pressure-side wall, which are connected to one another via a leading edge region and a trailing edge region. The cavity produced in this way has a crescent-shaped cross section curved orthogonally to the longitudinal direction and extends in the longitudinal direction rectilinearly between blade root and blade tip. Arranged in the cavity alternately on the suction-side wall and the pressure-side wall are cooling ribs, which enclose an acute angle relative to a projected center axis, the center axis resulting from an orthogonal projection of a plane which runs radially to the blade and perpendicularly to the inner sides of the walls and is arranged at the widest point of the cavity. The cooling fluid is therefore directed through the cooling passage in a kind of wave motion, the special shape of the ribs providing for good cooling efficiency.
Although this cooling concept has basically proved successful in practice, it encounters problems, in particular in the case of highly twisted and arched blade geometries, with regard to the uniformity of the temperature distribution at the blade body, which partly leads to inadmissibly high stresses.
Accordingly, one object of the invention, in attempting to avoid the disadvantages described, is to provide a novel cooling passage of a component subjected to high thermal loading of the type mentioned at the beginning which enables the cooling effect to be evened out further, even when the profile of the cavity is curved orthogonally to the longitudinal direction to a very pronounced degree, and in this way enables the demand for cooling medium to be minimized.
According to the invention, this is achieved in that, in a cooling passage according to the preamble of claim 1 having a profile of the cavity which is curved orthogonally to the longitudinal direction, the ribs are formed in such a way that, in each case locally with regard to the adjacent ribs of the opposite wall, they maintain a distance which corresponds to half a respective local rib spacing.
In concrete terms, the aim is thus for the ribs, at every point, to maintain a constant distance from the staggered ribs on the opposite wall, even in the case of a profile of the walls which is curved to a very pronounced degree. It is therefore necessary according to the invention to construct the individual ribs point by point in such a way that in each case two opposite ribs, at each point of the cavity in the direction of flow, are precisely at a distance from one another which corresponds to half the local spacing. Ribs which are no longer composed of two rectilinearly running legs but have a three-dimensionally curved form are obtained as a result. It is thus possible to even out the excitation of the heat transfer to the cooling fluid and to keep the pressure loss of the latter as low as possible on account of the fluidic optimization.
The ribs are preferably designed in such a way that the ratio of local rib spacing to a local rib height is essentially constant at each point of the rib. In this way, heat-transfer intensification which is kept constant as viewed in the direction of flow of the cooling fluid is obtained, this heat-transfer intensification resulting in a temperature distribution which does not vary greatly and thus in low thermal stresses in the component.
In a further, preferred variant, it is possible to vary the ratio of local rib height to a local cavity height and thereby realize local prominences of the rib. In this way, it is possible by means of secondary flows to force the cooling fluid preferably into marginal regions of the cavity, which are subjected to high thermal loading. Due to the secondary flows which occur, the cooling fluid from the marginal region is mixed with the cooling fluid in the core region, where the thermal load introduced from outside is low.
In addition, the ratio of local rib spacing to local rib height may advantageously be adapted in such a way that a uniform temperature is maintained in the longitudinal direction of the passage.
The combination of the two last-mentioned measures, for any cooling-passage profiles, leads to extreme evening-out of the temperature distribution in both the longitudinal direction and a transverse direction perpendicular thereto.
Since the cooling-fluid temperature in the direction of flow can increase considerably by absorption of heat energy, in particular in cooling passages having a comparatively large length, the ratio of local rib height to local cavity height is advantageously increased continuously in the direction of flow, as a result of which the temperature distribution in the longitudinal direction can be evened out still further.
For most applications, it is of advantage to form the ribs in each case from two curved rib segments and an apex, as a result of which wake zones in the side region can be prevented virtually completely. The heat input is often greatest precisely at these locations, so that intensification of the cooling effect is especially important there.
The ribs are particularly preferably designed in such a way that they can be depicted in an orthogonal projection as angles having rectilinearly running legs. For a multiplicity of applications occurring in practice, this represents a rib form which is to be considered optimal from the fluidic point of view and can be produced at an acceptable cost.
In this case, the angles are particularly preferably arranged symmetrically with regard to the center axis and parallel to one another.
Although the cooling concept according to the invention can in principle be used for any cooling-passage configurations, it is especially suitable for use on a coolable turbine blade or combustion-chamber wall of a gas turbine, which are subjected to thermal loads to a particularly high degree and which, with regard to the overall efficiency, on which the cooling demand has a considerable effect, must be continually improved for ecological and economic reasons.